1. Field of the Invention
The present invention relates to liquid crystal compounds, and more particularly to compositions containing the liquid crystal compounds.
2. Description of the Related Art
Reflective cholesteric liquid crystal displays have advantages of high contrast, high luminance, increased view angle, flicker-free image, and low power consumption, and high reflectivity for incident light of various wavelengths. Power consumption is 2% or less, an improvement on transparent liquid crystal displays.
In use, cholesteric liquid crystals retain image memory even without power supply, significantly reducing power consumption, consuming power only when refreshing, thereby demonstrating suitability for outdoor display or portable electronic devices.
Currently predominant is the transparent LCD type, essentially comprising a polarizer film, a color filter, and a backlight module, all contributing to higher manufacturing costs and device thickness than cholesteric liquid crystal display, which, with higher optical activity, selective light scattering and circular diachronism for liquid crystal composition, requires no polarizer film or color filter. Furthermore, the reflective cholesteric LCD without backlight module provides higher contrast and enhanced brightness under environmental light, even in sunlight. Compared to the conventional transparent LCD, the total manufacturing cost of cholesteric LCD can be reduced by about 30%.
Cholesteric liquid crystals can reflect light through Bragg reflection, because the cholesteric helix is a periodic structure. Light inside the material of wavelength equal to the pitch of the liquid crystal is reflected, provided it has circular polarization of the same orientation as the helix, and light propagation direction is along the helical axis. The bandwidth of wavelength for difference light reflected through common cholesteric liquid crystal molecule is, however, about 40˜50 nm, an unsuitable condition for LCD use with wavelength range for visible light from 400 to 700 nm. It is thus important to improve the structure of cholesteric liquid crystal molecules to meet the requirements of wider reflected wavelength range.
According to the Bragg reflection rule, the relationship between wavelength (λ) of cholesteric liquid crystal, average refraction ratio (nave) of thin film, and helical pitch (P) of cholesteric liquid crystal molecule is:λ=nave×P
The reflection bandwidth (Δλ) relates to the birefringence (Δn) and helical pitch (P) of cholesteric liquid crystal molecules, according to the relationship equation:Δλ=Δn×P
Accordingly, the reflected wavelength relates to the helical pitch (P) and optical anisotropic properties (birefringence) of the liquid crystal molecule. The cholesteric liquid crystal is composed of optically active chiral compound and nematic liquid crystal, with helical pitch controlled by the ratio therebetween. The relationship between helical pitch and amount of optically active chiral compound is:HTP=(P×C)−1
In the above relational formula, C represents the weight concentration of optically active chiral compound added to the liquid crystal composition and HTP helical twisting power, that is, the twisting and rotating ability of liquid crystal molecules. In general, the common chiral molecule has HTP below 15 μm−1.
H. -G. Kuball disclosed in 1995 that the value of HTP is based on the species of optically active functional group, number of chiral centers, and specific rotation. Thereby, the chiral compound can be designed to be more asymmetrical by represents of the accession of cyclic groups to increase HTP. Therefore, drawing large specific rotation, numerous chiral centers, and high helical twisting power cyclic groups into the structure of chiral compounds to improve reflection bandwidth of cholesteric liquid crystals is an important focus of research.
In addition, owing to the special helical structure of optically active chrial compound, solubility of chiral compound in the nematic liquid crystal deteriorates as weight concentration of chiral compound increases (such as more than 10 wt %), limiting the weight concentration of optically active chiral molecules.